The 3rd Generation Partnership Project (3GPP) is responsible for the standardization of the Universal Mobile Telecommunication System (UMTS) and Long Term Evolution (LTE). The 3GPP work on LTE is also referred to as Evolved Universal Terrestrial Access Network (E-UTRAN). LTE is a technology for realizing high-speed packet-based communication that can reach high data rates both in the downlink and in the uplink, and is thought of as a next generation mobile communication system relative to UMTS. Now that LTE (also sometimes referred to as “4G”) has been rolled out in implementations across the world, development attention has turned to the next generation of radiocommunication development, also referred to as the upcoming “5G” technologies.
After its initial rollout, the LTE specifications underwent further changes and improvements captured in various numbered releases. The most recent release for LTE was Release 14. In Release 12 of LTE, support for direct device-to-device (D2D) connectivity was first introduced. As the name suggests, direct D2D connectivity implies a direct radio link between devices, e.g., a direct radio link between two user equipments without a network node being involved in conveying payload data between the two user equipments. Given the relatively lower power of transmitters in mobile devices, D2D connectivity is typically only possible between devices in relatively close proximity to each other. Services based on D2D connectivity are therefore sometimes also referred to as proximity services or “ProSe”. Some illustrative examples of D2D communications are provided in the Detailed Description below.
One reason for introducing support for D2D connectivity in the LTE specifications was an explicitly expressed interest to use the LTE radio-access technology for public-safety-related communication services. For the public-safety use case it is seen as important, and in some cases even a requirement, to support at least a limited degree of local connectivity between devices even when there is no radiocommunication infrastructure available. Thus, support for direct D2D connectivity was seen as an important component to ensure LTE's fulfillment of all the requirements of the public-safety use case. However, support for D2D connectivity may also enable new types of commercial services, thus expanding the usability of the LTE radio-access technology in general.
Those skilled in the art will be familiar with the conventional terms of “uplink” and “downlink”, referring to radio resources which are assigned to enable communications from a user equipment to the network and from the network to a user equipment, respectively. However, for a direct device-to-device radio link, the notion of downlink and uplink transmission directions is not applicable. Instead, 3GPP has introduced the term “sidelink” to characterize the direct device-to-device link.
Since sidelink communications were not introduced into LTE from its inception, there have naturally been some design compromises which were required to enable sidelink communications to be added to an already mature radiocommunication design. For example, although LTE sidelink connectivity is technically possible in any portion of the normal cellular (LTE) spectrum, including both paired (FDD) and unpaired (TDD) spectrum, as well as both uplink and downlink spectrum, good co-existence between sidelink transmissions and normal cellular (downlink/uplink) transmissions in the same spectrum as well as existing regulatory requirements were key considerations in the design of LTE sidelink connectivity in Releases 12 and 13.
These considerations led to various design choices that characterize sidelink implementation in LTE. For example, sidelink communications were limited to uplink spectrum rather than also, or alternatively, using downlink spectrum. This choice was made based on, for example, both regulatory considerations (sidelink transmissions on the downlink spectrum would imply user equipment transmissions on spectrum allocated by regulatory bodies solely for transmissions by network devices), as well as ease of device implementation (from a device-implementation point-of-view, it is less complex to include additional receiver functionality (support for reception in an uplink band) compared to the additional transmitter functionality (needed in case sidelink connectivity would take place in downlink bands).
Another example involves the relatively simplistic nature of sidelink communications as implemented in LTE. In LTE Release 12 (and afterward), sidelink communications were understood to be fundamentally unidirectional in the sense that all sidelink transmissions are, essentially, broadcast transmissions with, for example, no associated control signaling in the opposite direction. There may of course be LTE sidelink transmissions from a device A received by a device B and, simultaneously, sidelink transmissions from device B received by device A. But these are then, radio-wise, completely independent transmissions.
Accordingly, with the advent of 5G radiocommunication systems, it would be desirable to develop sidelink communications that are designed to harmoniously co-exist with uplink and downlink communications from the inception of the new system, and which take advantage of being able to more freely balance complexity and performance of sidelink communications at the system's inception.